The use of optical fibers in communications is growing at an unprecedented rate. Low loss optical fibers which are produced by any one of several techniques may be assembled into ribbons which are then assembled into cables, or stranded into cables, or they may be enclosed singularly in a jacket and used in various ways in a central office, for example.
In order to assure that low loss fibers which are produced today are not diminished in their effectiveness in systems, the fibers must be connected through intermateable connectors which preserve those low losses. For fiber ribbons, connectors comprise grooved chips which hold a plurality of fibers of one ribbon in alignment with fibers of another ribbon. Such a connector is shown for example in U.S. Pat. No. 3,864,018 which issued on Feb. 4, 1975 in the name of C. M. Miller.
For single fiber cables, connections may be made through a connector which is referred to as a biconic connector. See U.S. Pats. Nos. 4,107,242 and 4,512,630 which issued on Aug. 15, 1978 and Apr. 23, 1985, in the name of P. K. Runge. That connector includes a housing in which is mounted a biconic alignment sleeve. The sleeve includes two truncated, conically shaped cavities which communicate with each other through a common plane which has the least diameter of each cavity. Each of two fibers to be connected is terminated with a plug comprising a cylindrical portion connected to a primary pedestal, a truncated, conically shaped end portion, which is adapted to be received in one of the cavities of the sleeve. The conically shaped surfaces of the plug and of the sleeve serve as alignment surfaces. The plug is urged into seated engagement with the wall defining the cavity in which it is received. The fiber extends through a passageway in the plug and has an end which terminates in a secondary pedestal of the plug. Generally, a plug is molded about an end portion of an optical fiber; however, there is a demand for plugs having passageways molded therein for the field termination of optical fibers.
Minimal loss between the connected fibers is achieved when the cores of fibers which are terminated by the plugs are aligned coaxially and when the longitudinal offset along the axes of the plugs is zero and fiber end faces, each of which is planar, contact in a common plane. Considering the size of the fibers, for example a single mode one with a core diameter of 8 microns and a cladding diameter of 125 microns, the task of providing conical plug and sleeve surfaces in order to meet alignment and end separation requirements is a formidable one. Generally, the plugs are molded from a transfer molding grade epoxy composition material. Although the surface tolerances which are achieved when molding the alignment sleeves and conic tapers are excellent, they are not sufficient to achieve consistently the desired alignment and end separation.
Problems arise because the opening in the end face of the pedestal and hence the optical fiber may not be centered with respect to the axis of revolution of the truncated, conically shaped portion. The axis of revolution of the conically shaped alignment surface of the end portion may also be referred to as the conical axis. As a result, the cores of the fibers terminated by two plugs held in the sleeve may have sufficient transverse or lateral offset to affect adversely the transmission of signals. Also, the centroidal axis of the end portion of the core of the fiber disposed in the passageway may not be coincident with the axis of revolution of the conically shaped end portion of the plug. Consequently, the light emitted from one fiber may not be parallel to the axis of the receiving fiber. This problem which is referred to as angular offset may also occur when the plug is molded about an end portion of an optical fiber. The angle between the fiber axis and the axis of revolution of the alignment surface of the plug end portion is commonly referred to as the exit angle of the plug.
Control of the exit angle as well as that of lateral offset is essential for achieving low loss connections and high yields in optical fiber connector manufacture. The control of these parameters insures that when two plugs are disposed in an alignment sleeve, not only will the end faces just touch, but that the fiber axes will be substantially coaxial.
A prior art method of reconfiguring an end portion of an optical connector into a passageway of which an optical fiber has not yet been inserted is disclosed in "Low-Loss field installable biconic connectors for single-mode fibers" by W. C. Young, et al., appearing in the OFC proceedings of Feb. 28 to Mar. 2, 1983. In this method, a light beam is projected through a passageway from a back end of the connector and a microscope is used to view the resulting back-lit boundary of an end portion of the passageway. As the plug is rotated on a support, an operator observes and adjusts manually the position of the plug until a disk of light bounded by the end portion of the passageway appears to be coaxial with the axis of rotation of the support. At that time, the alignment surface of the plug is reconfigured to become coaxial with the axis of rotation of the support and coincident with the apparent center of the boundary of the passageway as determined by the observer.
The hereinbefore-identified problems also have been overcome in the manufacture of factory terminated connectors as disclosed in copending, commonly assigned application Ser. No. 802,500 and application Ser. No. 802,492 (Pat. No. 4,721,357) filed on Nov. 27, 1985 in the names of R. P. Lyons, et al. and J. S. Kovalchick, et al., respectively. For plugs that have been terminated with an illuminated fiber having an end face polished perpendicular to the conic alignment surface of the plug, angular information may be precisely determined by the methods of hereinbefore identified copending application No. 802,492.
Machine vision-assisted methods as described in co-pending application Ser. No. 802,500 may be applied to assist in locating the centroid of the area defined by the boundary of the passageway. Unfortunately, the same technique applied to a back-lit passageway suffers from the effects of nonuniform illumination caused by defects or contamination of the passageway by foreign material or mold flash. As a result, both the angular and centroidal alignment of field mountable connectors by the axial viewing of light exiting the passageway is not as precise as when a fiber in the passageway is illuminated.
One disadvantage of such methods is that the location of the centroid of the light beam in the passageway in general is not the same as the centroid of a fiber end face which is intended to be disposed in the passageway when a single fiber optical cable is terminated with the connector. This is especially true with molded plastic plugs, in which the passageway exhibits deviations both in roundness and straightness due to variations in molding conditions such as mechanical stress on the wire mandrel which forms the passageway at the time molten plastic flows into a mold cavity. In addition, curvature and deviation from roundness of the passageway can cause shadows which obscure the true position of the boundary, resulting in errors during visual or machine vision determination of the boundary and the centroid of the passageway. A further disadvantage of prior art techniques is the difficulty in determining the angular axis of the passageway by viewing axially the light beam exiting the passageway.
Although the methods and apparatus disclosed and claimed in the above-identified applications have resulted in improved yields, there is a desire to achieve still further improvements in the area of field mountable connectors. What still is needed is a simple solution to the problem of providing production plugs at a relatively high yield for biconic connectors which may be used for on-site, i.e. field, termination of multi or single mode lightguide fibers. Each production plug must be such that a centroid of the overall transverse cross section of an optical fiber adjacent to an end face of the plug is coincident with the axis of revolution of the truncated, conically shaped surface of the plug, and such that the centroidal axis of the end portion of the fiber in the plug is substantially coaxial with the axis of revolution of the end portion of the plug. Desirably, the solution does not require additional elements or time in connection procedures, but instead involves the reconfiguration of molded plugs.